One prior art system utilizes an angular measuring device to measure the angle between two fight beams incident to an observation point. The light beams are produced by two light sources spaced apart on a vehicle. The angular difference between these two light beams near the observation point is the angular range and if the distance between the two light sources is known, the distance from the observer to the light sources can be calculated. Such a system is disclosed in U.S. Pat. Nos. 3,152,317 to D. W. Mayer and 3,340,763, to R. B. Power.
A system is disclosed in U.S. Pat. No. 5,177,462 to Kajiwara which utilizes a lens to receive a spot of reflected light and to focus an image of that spot on a light receiving element. The light emitting device and the lens are supported on the same vehicle and the distance to the vehicle reflecting the spot of emitted light is determined by triangulation using the distance between the light emitting device and the lens.
Yet another prior art system is disclosed in U.S. Pat. No. 5,249,027 to Mathur et al in which two spaced apart fight emitters on one vehicle transmit light trough a plurality of lens and onto a single sensor. As the patent points out this is the less desirable embodiment because the inter-vehicle distance is a function of the mounting distance between the two emitters. This is undesirable because each vehicle would have to rely on the correct (i.e., constant) separation distance between emitters on all other vehicles.